The primary objective of a hearing aid is to compensate for a user's hearing loss by amplifying and otherwise processing environment al sound received at an outwardly placed or ambient microphone of the hearing aid. Amplified or processed sound is emitted to the user's fully or partially occluded ear canal through a suitable miniature loudspeaker or receiver in manner where at least partial compensation of the user's specific hearing loss is accomplished.
However, mounting an ear mould or housing of the hearing aid in the user's ear canal introduces new imperfections. One such imperfection is occlusion, which is a phenomenon caused by full or partial physical blocking of the user's ear canal. The hearing aid user experiences occlusion as an unnatural exaggerated perception of low frequency components of his/hers own voice as well as excessive perception of jaw and mouth sounds which are conducted directly through bone and tissue of the user. Occlusion perception generally increases the more the hearing aid housing or ear mould blocks the ear canal and may vary between different styles of hearing aids such as in-the-ear (ITE), completely-in-the-canal (CIC) and behind the ear (BTE) and different characteristics of an ear mould.
The effect of occlusion and occlusion suppression on a hearing aid user is explained shortly below in a simplified situation in which the only sound sources considered are the receiver and the body conducted sound. In this simplified case of sound emission from a hearing aid, sound heard by the user will be a combination of a perceived or excess body conducted sound (BP=B−B′), and a receiver emitted sound (R), whereas a microphone in the ear canal would observe E=R+B=R+B′+BP, i.e. including the unnoticed or reference sound B′.
To give a hearing aid user an experience of unoccluded hearing, a ratio between body conducted sound and receiver generated or emitted sound must correspond to the ratio between body conducted sound and ear canal conducted sound for an unoccluded ear. If it was possible to isolate the perceived body conducted sound (BP), this sound could be emitted in opposite phase in the user's ear canal, with the effect of a perfect cancellation of the excess part of body conducted sound. This results in a perfect cancellation of occlusion sensation. However, in practice it is not possible to isolate the body conducted sound, and even less the perceived (i.e. “excess”) body conducted sound, but an ear canal microphone may be used to register the combination of body conducted sound and receiver emitted sound (E=R+B).
Assuming two receivers were placed in the hearing aid user's ear canal, one receiver could emit the ambient sound with an appropriate gain g (R1=g*A), and the other could subtract (i.e. emit in opposing phase) the registered ear canal sound with an appropriate gain f (R2=f*E=f*(R1+R2+B)=f*(g*A+R2+B) or R2=f(g*A+B)/(1−f)),
resulting in a perceived ear sound:(E=R1+B′+BP−R2)=g*A+B−f(g*A+B)/(1−f)=(1−(f/(1−f))*(g*A)+B)).
The occlusion suppression task then becomes to balance f and g, such that the sound heard by the user has the same ratio of body conducted sound to receiver emitted sound as the ratio between body conducted sound and ear canal conducted sound for an unoccluded ear. While this suppression task may appear simple, in practice it will involve a rather complex and calculation intensive optimization, which may not be desirable to perform in practice with current calculation power of Digital Signal Processors for hearing aids, especially considering the simplifications in the above explanation.
The practical implementation of an occlusion suppressor will typically not involve the use of two receivers, but rather be implemented in a device configured for subtraction of an electrical signal prior to output amplification.
The latter implementation will require an occlusion suppressor configured for processing the ear canal sound or sound pressure such that the after amplification the sum of the signal from a hearing loss compensation means and the occlusion suppressor will suppress the perceived body conducted sound, such that when the hearing aid is in normal operation, the user will perceive only the hearing loss compensated signal, without a perceived body conducted sound.
Hearing aid occlusion may be combated or suppressed by two methods; venting, and more recently, by signal processing. Venting may be implemented either as an acoustical vent comprising acoustical channels or conduits extending through the hearing aid housing or extending through the ear mould. Venting may alternatively be implemented as a so-called “open fitting” hearing aid with a loose fit in the user's ear. Both methods can be effective in reducing the user's perception of occlusion by allowing low frequency sound in the ear canal to escape to the surrounding environment through the vent. Venting is, however, accompanied by two significant adverse effects:                1) A suppression or attenuation of low frequency sound generated by the hearing aid;        2) An increased risk of acoustical feedback and hearing aid instability because of acoustical leakage through the vent to an ambient microphone(s) of the hearing aid.        
With respect to effect 1), low frequency components of the receiver sound is reduced by the same amount as the reduction in the occlusion level causing a reduction of both available low frequency gain and maximum undistorted output from the hearing aid at low frequencies. Since the individuals most affected by occlusion have mild loss to normal hearing at low frequencies, and thus don't need much, if any, gain for low frequencies, this is not necessarily a problem in itself.
With respect to effect 2), venting often leads to a requirement for feedback cancellation or suppression system to obtain a prescribed or target hearing aid gain. Feedback cancellation systems are accompanied by their own range of limitations and problems. Also, venting can give unpredictable results, sometimes producing much less occlusion reduction than expected. In some cases, venting may be supplemented by signal processing in suppression of occlusion in hearing aids.
Common for these approaches is that, an “ambient sound” received at the ambient microphone, is processed by a hearing loss processor to compensate for the hearing loss of a user to generate a desired sound, is combined with an compensation signal captured by a microphone in the user's partly or fully occluded ear canal volume in such a way that the sum of these signals suppresses the perceived excess body conducted sound.
While these approaches may be improvements over the previous approaches, Applicant of the subject application determines that they suffer from drawbacks, such as artefact sounds due to an unstable feedback loop or overload of an output amplifier or receiver enclosed in the feedback loop.